This study investigated the structural-phase, morphology, elemental composition, and tribological properties of coatings systems based on Al2O3, NiCr, and NiCr-Al2O3 fabricated by detonation-gas spraying method in a nitrogen/propane atmosphere. The coatings were characterised using SEM-EDX, XRD, Raman spectroscopy, and hardness measurements, and results analysed are presented. According to the experimental data, an optimal mode was selected that provides an increase in mechanical and tribological characteristics, and a method for detonation spraying surface hardening was also developed. Tribology tests have shown that the wear rates and friction coefficients of the coatings are highly dependent on the degree of filling of the gun.
TopIntroduction
Engineering materials are often exposed to mechanical influences, thermal radiation and aggressive chemical environments. The loss of performance often occurs due to wear, erosion and corrosion of the surface. The life of machine parts and mechanisms can be increased by applying protective coatings or modifying the surface layer. A promising technology is thermal spraying. Thermal spraying combines all the methods in which the coating on the surface of the substrate is formed from heated particles hitting it at high speed. This heating process involves the dispersion and transfer of condensed particles of the sprayed material by a gas or plasma stream to form a layer of the desired material on the substrate. At the same time, the three most important parameters: particle size, particle temperature and their velocity can vary greatly, depending on the spraying method. Under the general name thermal spraying (TS), the following methods are combined: gas flame spraying (wire and powder), supersonic gas flame spraying (wire and powder), cold powder spraying, detonation powder spraying, electric arc wire spraying and plasma powder spraying (atmospheric and vacuum). In essence, gas thermal spraying is very similar to welding, the difference lies in the functional purpose of the transferred material. The purpose of welding is to connect the construction elements of buildings, the purpose of thermal spraying is to protect the surface from corrosion, wear, etc. Gas-thermal methods have made it possible to obtain high-quality, durable coatings (Bannier et al., 2014; Dhakar et al., 2018; Ulianitsky et al., 2018; Zhang et al., 2008). For example, detonation spraying (DS) provides excellent adhesive strength, low porosity and a strong, dense and wear-resistant coating (Gallyamov et al., 2018; Thirumalaikumarasamy et al., 2014).
DS is the most promising and is carried out using a special detonation gun filled with an explosive gas mixture. Powder sprayed materials are used to form the coatings. The detonation process accelerates the powder particles to high speeds of up to 1300 m/s which melt and deposit on the sprayed surface. The advantage of DS over other gas-thermal methods (plasma and flame spraying, HVOF, HVAF) is the low porosity of the coatings obtained, high adhesion strength to the base of the workpiece (high adhesion), low thermal impact, which avoids unwanted thermal stresses, in addition, the sprayed parts are not deformed (Rakhadilov et al., 2019).
Detonation coatings are used in various fields of experimental science and technology. For example, for the oil and gas industry, the DS method has developed a technology for electrical insulation of threaded connections of drilling column elements. DS has proven to be effective in hardfacing applications, so it is used in the aircraft engine industry to treat the edges of the anti-vibration shelves of gas turbine blades in order to increase wear resistance. The high melting point (2044°C) at DS, combined with the specified strength characteristics and low porosity, makes it possible to apply heat-resistant and fire-resistant coatings made of Al2O3 to various metals, as well as to carbon and fiberglass. Also, DS technology is of interest in using in the production of ceramic coatings to ensure high dielectric strength, wear resistance of electrical insulating coatings (Algatti et al., 2017; Kaya et al., 2020; Ulianitsky et al., 2011; Zhu et al., 2020).
A combination of high values of adhesion strength, wear resistance, corrosion resistance, etc. are needed to improve the durability of detonation coatings operating under severe conditions. Multilayer coatings are very effective in enhancing performance particularly multilayer with a gradient structure which produces a smooth change in the chemical composition, structure and properties (physical, mechanical, etc.) along the coating thickness (Rakhadilov et al., 2020). A multilayer structure can be an effective method for improving the corrosion resistance of coatings. Interfaces between layers can act as protective barriers in aggressive environments (Pogrebnjak et al., 2009; Pogrebnjak & Tyurin, 2005).